> ## Documentation Index
> Fetch the complete documentation index at: https://docs.placematic.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Should I use Routing or Matrix?
> If you're writing a loop, you already know. One rule catches almost every error.
# Should I use Routing or Matrix?
**Short answer:** Routing returns a **path**. Matrix returns a **cost table**. If you're discarding the path and reading only `duration`, you asked the wrong question.
## One rule
**If you are writing a loop around a routing call, stop.**
You are building either a cost table (Matrix) or an itinerary (Tour Planning). Both exist. Both are cheaper.
## The arithmetic
An n×m cost table costs **n·m routing calls** or **1 matrix call**.
A 20-depot, 500-stop assignment problem is **10,000 calls or 1.**
That's a complexity difference, not a rate difference. **No pricing negotiation closes it.**
## Where the loop hides
* Nearest-driver assignment — one call per available driver
* Store locator ranking — one per candidate store
* Territory design — one per rep-base × unit centroid
* Delivery zone economics — one per zone per candidate
* Freight rating — one per lane, per quote, uncached
* Site selection — one per demand cell per candidate
**All matrices.**
## Why the mistake is systemic, not careless
**Routing is discoverable first.** It's what you reach for when you need a travel time.
**The loop works.** Correct answers. No error, no warning, no degradation.
**The failure is asymptotic.** At five stops it's imperceptible. Growth rates are invisible in a code review of the diff that introduced them.
**Matrix is async at scale.** Submit-poll-retrieve is more code and more state. Engineers optimizing for the shortest path to a green test choose synchronous.
**The response shape is unfamiliar.** Flat, row-major arrays — `travelTimes: [73, 1231, 983, 400]` for a 2×2 — not nested objects.
None of these are unreasonable. Naming the pattern explicitly in your architecture review is worth more than any amount of documentation.
## The inverse error
**Matrix returns no geometry.** If a driver follows it, you needed Routing.
## Size ceilings vary by mode
There is no single "HERE matrix size limit."
| Mode | Sync | Async | Live traffic + custom options |
| ---------------- | ------- | ------------- | ----------------------------- |
| Flexible | 15×100 | 15×100 | Yes |
| Region (≤400 km) | 500×500 | 10,000×10,000 | Yes |
| Profile | 500×500 | 10,000×10,000 | **No** |
**A truck-constrained matrix with live traffic runs in Flexible mode and caps at 15 × 100.**
Read the [spec](https://matrix.router.hereapi.com/v8/openapi) before architecting against any headline figure.
## Common misconceptions
**"I'll parallelize the loop."**
You've converted a cost problem into a `429` problem.
**"Matrix is only for huge problems."**
A 1×10 store ranking is one matrix call instead of ten routing calls. The ratio holds at every scale.
**"The flat array is just a format detail."**
Index it wrong and you get a **silently transposed matrix**. Every travel time is plausible. Every assignment is wrong. Nothing throws.
## Related
The full decision tree, and the indexing helper you write once.
Modes, async lifecycle, per-cell error codes.
Working code, with the indexing helper.
Where the matrix feeds the solver.
**HERE documentation:** [Matrix Routing v8 OpenAPI specification](https://matrix.router.hereapi.com/v8/openapi)
***
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